In conventional manner, the voltage characteristic of a storage cell as a function of its discharge capacity, e.g. expressed in Ah, does not vary significantly over a large majority of discharge characteristics, and it is therefore not possible to determine the state of charge of such a storage cell merely by measuring the voltage across its terminals.
The French patent application published under the number 2 685 780 describes, in particular, a method of measuring the state of charge of an electrochemical cell, the method being of the type that consists in applying a pulse stress to said cell and in observing its response to said stress in order to estimate its state of charge. That method consists in:
subjecting the electrochemical cell to a pulse of voltage at a value less than its nominal voltage, thereby causing it to deliver a discharge current, the value of the discharge current being such that the cell presents an unambiguous characteristic of internal impedance as a function of its state of charge; and
measuring said discharge current to estimate the state of charge of the electrochemical cell.
FIG. 1 shows an embodiment of circuit implementing that method.
The function of the circuit is to measure the state of charge of a storage cell 10. A control signal Ve is applied to the inverting input of an operational amplifier 11 whose non-inverting input is connected to the positive electrode of the storage cell 10. The negative electrode of the storage cell 32 is connected to ground. The output of the operational amplifier 11 is connected firstly to the cathode of a protective diode 12 having its anode connected to ground, and secondly to the base of switch means 13, constituted in this case by a transistor. Transistor 13 has its emitter connected to ground via a shunt resistor 14 that carries a current I when the transistor 13 is saturated, and its collector is connected to the non-inverting input of the operational amplifier 11. Its emitter is also connected to a circuit 15 for processing the voltage across the terminals of the resistor 14 and co-operating with a meter 16 for indicating the state of charge of the storage cell 10.
The operation of that circuit is described with reference to FIG. 2 which comprises two waveform diagrams showing the voltage Ve and the current I as a function of time.
The control signal Ve presents a voltage Vc that is greater than the voltage of the storage cell 10 which is written Vb so long as no pulse is being generated for the purpose of estimating the state of charge of the storage cell 10. The voltage at the output from the operational amplifier 11 is thus equal to zero or is negative. The transistor 13 is blocked and no current is carried by the shunt resistor 14. This state corresponds to a rest state for the system for measuring state of charge.
At a time t.sub.0 the signal Ve switches to a discharge voltage Vd. This sudden change in the control signal corresponds to a constant voltage pulse of duration T intended to enable the state of charge of the storage cell 10 to be measured. The output voltage from the amplifier 11 then switches to Vd (ignoring voltage drops), i.e. it switches to a voltage that is less than Vb. The voltage across the terminals of the storage cell 10 is then equal to Vd and considerable current passes through the shunt resistor 10. This current, written I.sub.0 varies in the same way as the admittance presented by the storage cell 10.
The waveform I=f(t) of FIG. 2 shows how the current I passing through the shunt resistor 14 varies. At time t.sub.0, the pulse discharge presents a peak, after which it stabilizes on a value I.sub.0. The current I.sub.0 may be very large, e.g. about 15 A for small capacity nickel-cadmium storage cells. By measuring the current I.sub.0 using the circuit 15, it is possible to determine the state of charge of the storage cell 10 and to display it on the meter 16. The circuit 15 may determine the admittance or the impedance of the storage cell 10. The duration .DELTA.T of the pulse is preferably sufficiently long to enable I.sub.0 to be measured at an instant in time that is remote from the beginning of the pulse. Variation in the current I.sub.0 at the beginning of the pulse is large so measuring I.sub.0 at a point A would be less accurate than measuring it at a point B which is preferably situated at an instant immediately before time t.sub.0 +.DELTA.T where the control signal Ve returns to Vc, thereby ending the state of charge measurement. The measurement instant corresponds to the moment at which the voltage across the terminals of the resistor 14 is measured.
Circuit of the above type has given good results for small format Ni-Cd type cells: the dynamic range of the signals is meaningful and the reproducibility thereof is satisfactory. In contrast, it turns out that applying it to same-size Ni-MH type cells or to 1.3 Ah VECS type cells is not satisfactory since their characteristics of current delivered as a function of residual capacity presents a plateau, as shown in FIG. 3.
FIG. 3 shows the characteristic 30 of current I (expressed in amps) delivered by a battery of storage cells made up of five VECS type Ni-Cd cells having a capacity of 1300 mAh, as a function of its residual capacity Cr (expressed in mAh).
It can be seen that for states of charge lying in the range 30% to 80% of nominal capacity, the characteristic 30 presents a plateau, thereby making it difficult to estimate state of charge within this range. An ideal characteristic is represented by straight line 31, and an optimum estimate of a state of charge would assume that the characteristic I=f(Cr) for the storage cell under test is as close as possible to said straight line 31. The appearance of the characteristic 30 is due to parasitic resistance present in the storage cell under test, which parasitic resistance is shown in FIG. 4.
FIG. 4 is an equivalent diagram of an electrochemical cell 10.
This equivalent diagram comprises a series connection of a voltage source U, an internal resistance Ri, which is a function of the state of charge of the electrochemical cell, and a parasitic resistance Rp. The parasitic resistance Rp gives rise to the plateau in FIG. 3 and it is due to parameters such as the structure of the cell, the nature of its electrochemical couple, or the connections between cells (for a battery of storage cells connected in series).